1. Field of the Invention
This invention relates to a metal-containing composition that can be used effectively for manufacturing an electron-emitting device comprising an electroconductive film containing therein an electron-emitting region and arranged between a pair of device electrodes and it also relates to an electron-emitting device formed by using such a composition, an electron source comprising a number of such devices and an image-forming apparatus realized by using such an electron source.
2. Related Background Art
The use of surface conduction electron-emitting devices in a cold cathode type electron source is known. A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow therethrough in parallel with the film surface. While Elinson proposes the use of SnO.sub.2 thin film for a device of this type, the use of Au thin film is proposed in [G. Dittmer: "Thin Solid Films", 9, 317 (1972)] whereas the use of In.sub.2 O.sub.3 /SnO.sub.2 and that of carbon thin film are discussed respectively in [M. Hartwell and C. G. Fonstad: "IEEE Trans. ED Conf.", 519 (1975)] and [H. Araki et al.: "Vacuum", Vol. 26, No. 1, p. 22 (1983)].
FIG. 17 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 17, reference numeral 171 denotes a substrate. Reference numeral 174 denotes an electroconductive film, part of which eventually makes an electron-emitting region 173 when it is subjected to an electrically energizing process referred to as "energization forming" as will be described hereinafter. In FIG. 17, the device electrode has a length L of 0.5 to 1 mm and a width W of 0.1 mm.
Conventionally, an electron emitting region 173 is produced in a surface conduction electron-emitting device by subjecting the electroconductive film for forming an electron-emitting region of the device to a current conduction treatment, which is referred to as "energization forming". In an energization forming process, a voltage is applied to the opposite ends of the electroconductive thin film for forming an electron-emitting region by way of the device electrodes to partly destroy, deform or transform the film and produce an electron-emitting region 173 which is electrically highly resistive. A fissure or fissures may be produced in the electroconductive film 174 as a result energization forming to make an electron-emitting region 173 of fissure so that electrons may be emitted from the fissure itself or from an area surrounding the fissure.
Note that, once subjected to an energization forming process, a surface conduction electron-emitting device comes to emit electrons from its electron emitting region 173 whenever an appropriate voltage is applied to the electroconductive film 124 to make an electric current run through the device.
Since a surface conduction electron-emitting device having a configuration as described above is structurally simple, a large number of such devices can advantageously be arranged over a large area. Efforts have been made to exploit this advantage and the devices proposed to exploit this characteristic feature of surface conduction electron-emitting device include charged beam sources and display apparatuses. Japanese Patent Applications Laid-Open Nos. 64-31332, 1-283749 and 2-257552 proposes an electron source comprising a large number of surface conduction electron-emitting devices arranged in parallel rows, where the devices of each row are commonly wired in a ladder-like arrangement. While flat-type displays using a liquid crystal have come into the mainstream of image-forming apparatuses to push out, at least partly, CRT displays, the liquid crystal display has a drawback of requiring the use of a back light because it is not of emission type and does not beam unless irradiated with light. Therefore, there is a consistent demand for emission type displays. The U.S. Pat. No. 5,066,883 discloses an image-forming apparatus realized by combining an electron source comprising a large number of surface conduction electron-emitting devices and an fluorescent body that emits visible light when irradiated with electrons emitted from the electron source.
An electroconductive film for forming an electron-emitting region is typically produced by depositing an electroconductive material on an insulating substrate directly by means of an appropriate deposition technique such evaporation or sputtering. An electroconductive film for forming an electron-emitting region may also be produced by applying, drying and baking a solution of a metal compound to remove the non-metal components of the solution by pyrolysis and form a thin film of metal or metal oxide. The latter technique is advantageous for producing a large number of devices on a substrate having a large surface area because it does not involve the use of a vacuum apparatus.
Materials that can be used for forming an electroconductive film of metal or a metal compound by way of an liquid applying, drying and baking process include a liquid containing a metal resinate or a compound of precious metal such as gold and resin and a solution prepared by dissolving an organic complex of organic amine and transition metal into an organic solvent. In short, electron-emitting devices can be manufactured from various different solutions.
It is well known, on the other hand, that many halides and oxyacid salts of transition metals are water soluble and produce corresponding metals or metal oxides by pyrolysis when heated to high temperature.
However, known metal compositions that can be used for manufacturing electron-emitting devices comprising an electroconductive film that contains an electron-emitting region such as surface conduction electron-emitting devices are accompanied by a number of problems as will be described hereinafter.
While it is true that many halides and oxyacid salts of transition metals are water soluble and produce corresponding metals or metal oxides by pyrolysis when heated to high temperature, the temperature for pyrolyzing such compounds is typically higher than 800.degree. C., although it is not desirable to prepare electroconductive films for surface conduction electron-emitting devices by pyrolysis involving such high temperature. A number of surface conduction electron-emitting devices are formed on the surface of an appropriate substrate that carries a pattern of wires for wiring the devices. In other words, if such a pattern of wires is prepared on the substrate along with the electrodes of surface conduction electron-emitting devices before the electroconductive films of the devices are formed, the conditions for producing the electroconductive films by baking have to be carefully selected in order to avoid damages that may be given rise to the patterned wires and/or the electrodes by heat. More specifically, if the substrate is a silicon wafer or a glass substrate, the heating and baking process for producing electroconductive films on the substrate has to be conducted at temperature lower than 600.degree. C., preferably at about 500.degree. C., where the material of the wires such as copper or silver is not thermally degraded. Thus, any materials that have to be heated to temperature higher than 500.degree. C. for producing electroconductive films may not suitably be used for manufacturing surface conduction electron-emitting devices. Aqueous solutions of halides or oxyacid salts of transition metals that require high baking temperature may not be used for preparing electroconductive films in the manufacture of surface conduction electron-emitting devices if such compounds are easily soluble to water.
Meanwhile, a number of organic metal complexes of a metal resinate or organic amine and a transition metal that may be easily decomposed at relatively low temperature lower than 500.degree. C. are known. Since most of the organic metal compounds that decompose at relatively low temperature are easily soluble in ordinary organic solvents, they are typically dispersed or dissolved in an organic solvent for use. When a compound containing a metal to be used for forming a thin film is dispersed into an appropriate solvent to produce a liquid material, which is then applied to the surface of a substrate and baked to produce an electroconductive film for a surface conduction electron-emitting device, the solvent is preferably harmless to human and poorly inflammable from the view point of the environment and security of the process of manufacturing electron-emitting devices. In other words, the use of water as a solvent is preferable for the security of the process of manufacturing electron-emitting devices. Unfortunately, the organic metal compounds that are decomposed at relatively low temperature and hence can be used for manufacturing electroconductive films of surface conduction electron-emitting devices are mostly not sufficiently water soluble and it has been difficult to date to obtain an aqueous solution containing a metal compound to such a ratio that is appropriate for manufacturing electroconductive films of surface conduction electron-emitting devices.
Some of the organic metal complexes of an organic amine and a transition metal that are decomposed at relatively low temperature can evaporate or sublimate when heated for baking. If such an organic metal complex is used in the process of manufacturing surface conduction electron-emitting devices and applied to the substrate at a given rate, part of the metal can be lost while the substrate is baked and the amount of the metal left on the substrate after baking is dependent on the baking conditions and hence unstable and unreliable. Additionally, the vapor of a transition metal compound generated in the process of manufacturing surface conduction electron-emitting devices can damage the environment and hence undesirable.
Some of the organic metal complexes of an organic amine and a transition metal that are decomposed at relatively low temperature can form a crystalline structure having a size of several micrometers or more when dissolved into an organic solvent and applied to the surface of a substrate. When the applied solution is baked and dried, the pattern of the crystal can be left on the electroconductive film. Such an uncontrolled pattern can obviously obstruct the formation of an electroconductive film having a uniform thickness and a uniform electric resistance particularly when combined with the above problem of evaporation of the organic metal complex.
Many organic acid salts of metals such as metal carboxylates decompose at temperature under 500.degree. C. to produce metals and/or metal compounds. If the molecule of an organic salt of a metal has a relatively small number of carbon atoms, it can more often than not dissolve into water. Meanwhile, an electron-emitting device has to operate stably for a long period of time. Therefore, the electroconductive film of the surface conduction has to be made of a material that is thermally and structurally stable and hardly change with time in the operating environment. Thus, the metal component of the electroconductive film of a surface conduction electron-emitting device has to be selected from chemically and thermally stable metals having a high melting point. However, many organic acid salts of metals, particularly metal carboxylates, do not satisfactorily dissolve into water and are often accompanied by the problem of evaporation or sublimation as they only partly dissolve into water if heated.
Electron-emitting devices can be arranged on a substrate in large numbers in order to form an electron source for an image-forming apparatus. For such an application, a large number of identical electron-emitting devices have to be formed at regular intervals over a large area on a highly reproducible basis. The technique of photolithography has been popularly used to form a large number of devices on a substrate as in the case of manufacturing semiconductors. However, this technique is not suited to produce a large number of devices on a substrate having a large surface area and it is often costly.
A technique of applying a solution that contains a metal compound little by little on a given pattern on a substrate and baking it to form small pieces of electroconductive film that are arranged according to the given pattern may be used in place of photolithography in order to produce a large number of identical electron-emitting devices on a substrate on a highly reproducible basis. An ink-jet system may be effectively used for applying a solution on a substrate. However, this technique is accompanied by the problem of crystallization and deposition of the metal compound that can take place during the ink-jet operation and/or in the time interval before the next operation starts. The net result will then be electroconductive films having a remarkably uneven thickness and electron-emitting devices that would not operate uniformly.
There has been proposed the use of a bubble-jet system, which is a type of ink-jet system, for manufacturing electroconductive films. (See, inter alia, Japanese Patent Applications Laid-Open Nos. 6-313439 and 6-313440.) A bubble-jet system can produce and apply a fine drop of liquid efficiently and accurately in a highly controlled manner and hence is effective for the above purpose. However, an ink-jet system is most effectively used with an aqueous solution of an organic metal compound in view of the durability of the nozzle head and the generation of fine drops. Conversely, it is not suited for an organic metal compound that hardly dissolve into water. This drawback on the part of ink-jet is still to be dissolved.
Printing may provide a less costly method for producing device electrodes for electron-emitting devices if compared with a technique using evaporation, sputtering and lithography in combination. However, a thin film prepared by printing shows a low film density if compared with a film produced by evaporation so that, when a solution is applied to the electrodes to produce an electroconductive film for forming an electron-emitting region, it can permeate, at least partly, into the electrodes and become lost. Then, the result will be an unintended and uneven thickness of the electroconductive film after baking. Thus, if a large number of such electroconductive films are produced on a same substrate, they operate very unevenly for electron emission to the detriment of the performance the electron source formed by the electroconductive films.
As described above, a metal-containing solution is desirably applied to a substrate according to a given pattern before they are baked to become small pieces of electroconductive film for electron-emitting devices. However, the inventors of the present invention have found that, if such a solution is applied to a substrate, it does not necessarily show an intended pattern nor a uniform film thickness after it is baked.
As a result of intensive research efforts on the performance various metal-containing compositions, the inventors of the present invention have discovered that a desired pattern cannot be obtained mainly due to either one of two phenomena. Firstly, the solution applied to the substrate can be repelled by the substrate and drops of the solution can be formed on the substrate to deform the pattern. Secondly and conversely, the solution applied to the substrate can excessively adhere to the substrate to wet unintended areas of the latter. It is obvious that either of these phenomena appears as a function of the cohesiveness of the solution or the adhesiveness of the solution relative to the substrate. Therefore, it may conceivably be possible to select a liquid composition that shows an optimum contact angle relative to the substrate by observing the contact angle of the solution and the substrate. However, as a result of a further study, it has been found that a solution that shows an optimum contact angle relative to a substrate does not necessarily provide a desired pattern of electroconductive film.
Additionally, the surface of the substrate on which electron-emitting devices are formed is not necessarily flat and smooth because wires and electrodes for supplying power to the devices are already there. When a metal-containing composition is applied to the surface of an insulating substrate that already carries device electrodes, the metal-containing composition has to adhere appropriately to both the surface of the metal electrodes and that of the insulating substrate. However, since the metal surface and the surface of an insulating substrate have respective properties that are so different from each other, it is not easy to find an appropriate metal-containing composition that adheres appropriately to both of them.